Generally, a vehicles front wheels rotates in association with the steering wheel. That is, when the steering wheel is rotated leftward, the vehicle wheels rotate leftward. When the steering wheel is rotated rightward, the vehicle wheels rotate rightward.
However, since the vehicle wheels are in contact with a road surface, a rotational angle of the steering wheel may vary due to friction between the vehicle wheels and the road surface. That is, when the steering system of the vehicle steers the vehicle wheels by rotating the steering wheel, friction operates between the vehicle wheels and the road surface, causing a difference in a rotational angle between the steering wheel and the vehicle wheels. Accordingly, a rotational deflection is generated.
Taking this into consideration, most steering systems adopt an electronic power steering (EPS) system that measures rotational deflection using a torque sensor and supplies a dedicated rotational force to an output shaft corresponding to the measured rotational deflection, thereby compensating for the rotational deflection.
Since the EPS system measures the rotational deflection between a steering wheel and a wheel using the torque sensor and rotates the vehicle wheels using a dedicated power unit by as much as the measured rotational deflection, the vehicle may be steered safely and accurately in a desired direction.
A known EPS system is shown in FIG. 1 with a steering column having an input shaft 10 connected with a steering wheel, an output shaft 20 connected with a pinion gear 21 which is meshed with a rack bar of a tie rod of a wheel, and a torsion bar 30 coaxially connecting the input shaft 10 with the output shaft 20.
In the known EPS system, when resistance between the vehicle wheels and a road surface is great, the input shaft 10 is rotated more than the output shaft 20. Accordingly, the torsion bar 30 is twisted, which may be detected using a torque sensor with magnet detection rings 41, 42, and 43 and an electrical signal is input to an electronic control unit (ECU). The input electrical signal is calculated in the ECU, thereby driving an auxiliary power unit (APU). Thus, deficiency of the rotational angle of the output shaft 20 may be compensated.
The torque sensor generally used in the EPS system may include a contact type sensor which directly measures deformation of the torsion bar, such as a strain gauge and a potentiometer, and a non-contact type sensor which indirectly measures deformation of the torsion bar using a magnetic or optical method.
FIG. 2 shows a known non-contact torque sensor that includes a ring-type rotor 50 connected with an input shaft and on which a multi pole magnet 51 including N poles and S poles alternately polarized in a circumferential direction is arranged, and stators 52 and 53 connected with an output shaft and divided into an upper part and a lower part to respectively include protrusions 52a and 53a corresponding to the multi pole magnet 51 and recessed sections 52b and 53b relatively recessed with respect to the protrusions 52a and 53a. A collector unit 30 may be provided between the stators 52 and 53, to form a circuit for flow of a magnetic flux. In the known non-contact torque sensor, when a torsion bar is twisted by rotation of a steering wheel, the rotor 50 and the stators 52 and 53 are rotated relative to one another. At this time, relative positions of the multi pole magnet 51 and the protrusions 52a and 52b are changed. Therefore, the known non-contact torque sensor may measure a rotational deflection of an input shaft and an output shaft by detecting density of a magnetic flux flowing to the collector unit 30.
FIGS. 3A to 3C show a flow of a magnetic flux according to positions of the protrusions and the rotor of the known non-contact torque sensor shown in FIG. 2. As shown in FIG. 3A, when the protrusions 52a and 52b overlap with the N poles and the S poles by the same area, almost no magnetic flux flows to the collector unit 30.
However, when the N poles overlap the protrusion 52a of an upper stator 52 while the S poles overlap the protrusion 53a of a lower stator 53 as shown in FIG. 3B, a magnetic flux emitted from the N poles flows to the upper stator 52 through the protrusion 52a of the upper stator 52 and the magnetic flux flowed to the upper stator 52 flows to the lower stator 53 through the collector unit 30 and then is collected to the S poles. That is, in the case as shown in FIG. 3B, the magnetic flux flowing in the collector unit 30 may flow downward.
In addition, as shown in FIG. 3C, when the S poles overlap the protrusion 52a of the upper stator 52 while the N poles overlap the protrusion 53a of the lower stator 53, a magnetic flux emitted from the N poles flows to the lower stator 53 through the protrusion 53a of the lower stator 53. The magnetic flux flowed to the lower stator 53 flows to the upper stator 52 through the collector unit 30 and then is collected to the S poles. That is, in the case as shown in FIG. 3C, the magnetic flux flowing in the collector unit 30 may flow upward.
Thus, in a known magnetic sensor provided to the collector unit 30 to measure density of the magnetic flux, directions of the magnetic flux alternate upward and downward according to rotation of the rotor. Therefore, a hysteresis loss is increased.